In a legged mobile robot with a plurality of legs, each of the legs is configured by sequentially connecting a plurality of link members through a plurality of joints from a body. For example, in a biped mobile robot with two legs like a human, each of the legs is configured by sequentially connecting the link members, which correspond to a thigh portion, a crus portion, and a foot portion, through a hip joint, a knee joint, and an ankle joint, respectively, from a body of the robot. In addition, in the legged mobile robot of this kind, a motion of each of the legs for moving the robot is produced by applying a driving force (torque) to each of the joints of each of the legs by using a joint actuator such as an electric motor.
Incidentally, in the mobile robot of this kind, when, for example, a movement speed thereof is increased, forces (moment) acting on the joints of each of the legs are likely to be relatively large in a foot landing state of each of the legs (a state of a supporting leg period of each of the legs), due to floor reaction forces or the like. Consequently, driving forces (torque), which should be generated to the joint actuators to resist the forces, are likely to be relatively large. For example, in a case of allowing the biped mobile robot to run with a gait (a motion pattern of legs) similar to the gait of a running human, the driving force to be generated to the joint actuator of the knee joint becomes large, particularly in a supporting leg period of each of the legs, according to the knowledge of the inventor and the like. In this case, when the joint actuator is an electric motor, the aforementioned driving force is generated by a regenerative operation or a powering operation of the electric motor. With any of these operations, however, it is required to energize the electric motor or a power source such as a battery with a high current. Thus, an energy loss by Joule heat or the like is likely to be large. Further, since the electric motor with a large capacity is required, the size and weight of the electric motor become large.
Meanwhile, as disclosed in Japanese Patent Laid-Open Publication No. 2001-198864 (especially FIG. 9 of this publication), for example, a biped mobile robot is known in which a spring is provided between two link members (a thigh portion and a crus portion) connected by a knee joint of each leg.
While moving horizontally, this biped mobile robot converts kinetic energy in the horizontal direction of the robot into elastic energy of the spring and stores the elastic energy, thus producing a jumping motion of the robot by the use of the elastic energy. In the biped mobile robot provided with the springs as above, a part of a driving force to be generated in each of the knee joints is provided by the elastic force of the spring during a part of the period when the robot is in the running motion or the like. Thus, a burden on the joint actuators of the knee joints can be reduced. However, in this biped mobile robot, the elastic force of the spring is always acting between the thigh portion and the crus portion of each of the legs. Therefore, while the biped mobile robot is moving, a situation occurs where the elastic force of the spring acts in an opposite direction to the driving force which should be generated in each of the knee joints. In such a situation, a driving force generated to the joint actuator of each of the knee joints becomes unnecessarily large. As a result, it becomes difficult to improve utilization efficiency of the total energy of the robot.
The present invention was accomplished in light of the above-described circumstances, and it is an object of the present invention to provide a leg joint assist device for a legged mobile robot, which is enabled to reduce burdens on joint actuators as necessary and to stably ensure favorable utilization efficiency of energy.